Out of the box, Play provides a mechanism for runtime dependency injection - that is, dependency injection where dependencies aren’t wired until runtime. This approach has both advantages and disadvantages, the main advantages being around minimisation of boilerplate code, the main disadvantage being that the construction of the application is not validated at compile time.

An alternative approach that is popular in Scala development is to use compile time dependency injection. At its simplest, compile time DI can be achieved by manually constructing and wiring dependencies. Other more advanced techniques and tools exist, such as macro based autowiring tools, implicit auto wiring techniques, and various forms of the cake pattern. All of these can be easily implemented on top of constructors and manual wiring, so Play’s support for compile time dependency injection is provided by providing public constructors and factory methods as API.

In addition to providing public constructors and factory methods, all of Play’s out of the box modules provide some traits that implement a lightweight form of the cake pattern, for convenience. These are built on top of the public constructors, and are completely optional. In some applications, they will not be appropriate to use, but in many applications, they will be a very convenient mechanism to wiring the components provided by Play. These traits follow a naming convention of ending the trait name with Components, so for example, the default HikariCP based implementation of the DB API provides a trait called HikariCPComponents.

In the examples below, we will show how to wire a Play application manually using the built in component helper traits. By reading the source code of these traits it should be trivial to adapt this to any compile time dependency injection technique you please.

One aim of dependency injection is to eliminate global state, such as singletons. Play 2 was designed with an assumption of global state. Play 3 will hopefully remove this global state, however that is a major breaking task. In the meantime, Play will be a bit of a hybrid state, with some parts not using global state, and other parts using global state.

By using dependency injection throughout your application, you should be able to ensure though that your components can be tested in isolation, not requiring starting an entire application to run a single test.

Every application that runs on the JVM needs an entry point that is loaded by reflection - even if your application starts itself, the main class is still loaded by reflection, and its main method is located and invoked using reflection.

In Play’s dev mode, the JVM and HTTP server used by Play must be kept running between restarts of your application. To implement this, Play provides an ApplicationLoader trait that you can implement. The application loader is constructed and invoked every time the application is reloaded, to load the application.

This trait’s load method takes as an argument the application loader Context, which contains all the components required by a Play application that outlive the application itself and cannot be constructed by the application itself. A number of these components exist specifically for the purposes of providing functionality in dev mode, for example, the source mapper allows the Play error handlers to render the source code of the place that an exception was thrown.

The simplest implementation of this can be provided by extending the Play BuiltInComponentsFromContext abstract class. This class takes the context, and provides all the built in components, based on that context. The only thing you need to provide is a router for Play to route requests to. Below is the simplest application that can be created in this way, using a null router:

By default Play will generate a static router that requires all of your actions to be objects. Play however also supports generating a router than can be dependency injected, this can be enabled by adding the following configuration to your build.sbt:

routesGenerator := InjectedRoutesGenerator

When you do this, Play will generate a router with a constructor that accepts each of the controllers and included routers from your routes file, in the order they appear in your routes file. The routers constructor will also, as its first argument, accept an HttpErrorHandler, which is used to handle parameter binding errors. The primary constructor will also accept a prefix String as the last argument, but an overloaded constructor that defaults this to "/" will also be provided.

Note that the naming of the parameters is intentionally not well defined (and in fact the index that is appended to them is random, depending on hash map ordering), so you should not depend on the names of these parameters.

As described before, Play provides a number of helper traits for wiring in other components. For example, if you wanted to use the messages module, you can mix in I18nComponents into your components cake, like so: